Interaction of wide band gap single crystals with 248 nm excimer laser irradiation. VIII. Laser desorption of molecular ions from MgO

Abstract

We present unambiguous evidence for direct, nonthermal laser desorption of molecular ions from the surface of an ionic wide band gap material, single crystal MgO. Using time resolved mass spectroscopy, we measure positive ions emitted during pulsed excimer laser irradiation at 248 nm. Species observed (besides singly and doubly charged Mg) include: Mg2+, MgO+, MgO2+ , Mg2O+ and (MgO)2+. The kinetic energies determined from time of flight measurements are species dependent, nearly independent of fluence, and range from 5 to 9 eV. The ion intensities display strongly nonlinear fluence dependencies, proportional to the fluence to the nth power, where n ranges from 3 to 4. These kinetic energies and fluence dependences support a defect mediated photoelectronic desorption mechanism. Emission is attributed to molecular ion precursors adsorbed at special sites associated with surface electron traps. Emission occurs when the electron trap is photoionized via the sequential absorption of 3–4 photons.

The interaction of 193-nm excimer laser irradiation with single-crystal zinc oxide: Positive ion emission

“…We have previously shown that laser-induced ion emission from wide bandgap insulators originates from surface defect sites where a positive ion is sorbed on top of a surface electron trap. [1][2][3][4][5][6][7][8][9] We suggest that the corresponding defect configuration for semiconducting ZnO is a zinc ion sorbed on top of a surface oxygen vacancy. When the electron trap is photoionized by the laser, the electrostatic force on the sorbed positive ion changes sign and the ion is ejected with significant kinetic energy.…”

“…This reflects an intermediate case between metals, where direct ion emission is strongly hindered by the high probability of neutralization, 17 and insulators, where neutralization is rare and intense positive ion emissions are often observed. [1][2][3][4][5][6][7][8][9] We have observed Zn þ and O þ emission from singlecrystal Zn ð10 10Þ surfaces exposed to 193-nm excimer laser radiation. At fluences below about 150 mJ/cm 2 , Zn þ is the dominant positive ion.…”

Positive ion emission from oxidized aluminum during ultraviolet excimer laser irradiation

“…When the electron trap is photoionized by the laser, the electrostatic force on the sorbed positive ion changes sign and the ion is ejected with significant kinetic energy. [1][2][3][4][5][6][7]25 The final kinetic energy of the emitted ion depends primarily on the defect geometry, and often exceeds the photon energy. Although prolonged irradiation normally consumes defects and reduces the ion intensities, the ion kinetic energies change very little.…”

Surface analysis by laser-induced desorption time-of-flight mass spectrometry

“…An electronic shutter was placed in the beam path to pick out single laser pulses from the low frequency Q-switched train (the time interval between two consecutive pulses was >3 s) to eliminate residual effect from the previous pulse. 19 The timing between the firing of laser and ionization of the neutral species was synchronized to obtain optimum signal. TOFMS data was recorded at a time resolution of 10 ns by a Wavepro 940 digital oscilloscope (LeCroy).…”